Vapor deposition on electrostatically tensioned foil

ABSTRACT

A method of vapor depositing metal on plastic foil which maintains the foil smooth so as to prevent creases and wherein the foil is passed over a cooling drum which is covered with an insulating layer and wherein an electrostatic charge is maintained between the foil and the cooling drum so as to cause attraction between the foil and the cooling drum thus assuring good contact and smooth foil arrangement on the drum as the foil is metalized. Simultaneous testing of weak spots in the foil are tested. The insulating layer on the drum is broken into small areas and the product of its resistance and its thickness is selected so that it does not exceed one-fifth of the product of the resistance and thickness of the foil.

Kessler et al.

Dec. 9, 1975 VAPOR DEPOSITION ON ELECTROSTATIC ALLY TENSIONED FOILInventors: Hartmut Kessler; Hubert Kraus,

both of Regensburg, Germany Assignee: Siemens Aktieng esellschaft,Berlin &

Munich, Germany Filed: Jan. 9, 1974 Appl. No.: 431,851

Foreign Application Priority Data Mar. 7, 1973 Germany 2311217 US. Cl.118/49.1; 226/94; 226/195 Int. C1. C23C 13/08 Field of Search 118/8, 7,4849.1;

References Cited UNITED STATES PATENTS 4/1961 Fountain 118/8 UX 12/1962Owens 226/94 X 3/1963 Vaccaro 226/94X 1/1968 Bullwinkel 324/54 FOREIGNPATENTS OR APPLICATIONS 715,013 9/1954 United Kingdom 226/94 864,7854/1961 United Kingdom 324/54 Primary Examiner-Morris Kaplan Attorney,Agent, or FirmI-Iill, Gross, Simpson, Van Santen, Steadman, Chiara &Simpson ABSTRACT A method of vapor depositing metal on plastic foilwhich maintains the foil smooth so as to prevent creases and wherein thefoil is passed over a cooling drum which is covered with an insulatinglayer and wherein an electrostatic charge is maintained between the foiland the cooling drum so as to cause attraction between the foil and thecooling drum thus assuring good contact and smooth foil arrangement onthe drum as the foil is metalized. Simultaneous testing of weak spots inthe foil are tested. The insulating layer on the drum is broken into,small areas and the product of its resistance and its thickness isselected so that it does not exceed one-fifth of the product of theresistance and thickness of the foil.

3 Claims, 1 Drawing Figure VOLTAGE SOURCE US. Patent Dec. 9, 1975VOLTAGE SOURCE VAPOR DEPOSITION ON ELECTROSTATICALLY TENSIONED FOILBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates in general to a method for depositing metal on plastic foils ina vacuum chamber which can be used for the regeneration of capacitorfoils.

2. Description of the Prior Art It has been known in the past to depositmetal vapors on plastic foil, but creases and folding of the foil haveoccurred as they pass through the metalizing region.

SUMMARY OF THE INVENTION The present invention provides a cooling drumfilled with a cooling substance and which has an outer metal surfaceover which is provided a layer of insulation material which engages oneside of the plastic foil upon which the metalized layer is formed. Anelectric field is applied between the metalized layer and the coolingdrum so as to attract the plastic film and hold it smooth and unwrinkledunder the drum as vapor deposit of metal occurs on the opposite surfaceof the plastic foil. The electric field will also cause breakdowns atholes or weak points of the foil which are clearly distinguishable fromcommon burnouts which occur in air under pressure in that a much largerarea of the metal coating is effected. The inventors have discoveredthat by incorporating a special barrier resistance in the current supplyline that the area of the burnout regions can be controlled so that theydo not cover as large an area. Due to the high number of breakdowns, thevoltage will decrease behind a normal barrier resistance because of thecurrent required to charge the insulating areas where breakdown hasprevisouly occurred.

The present invention allows the maintaining of the optimum voltagebetween the coating and the cooling drum for the vapor deposition ofmetal in that the present invention provides for the maintenance of theoptimum voltage necessary for preventing heat creasesand wrinkles in theplastic film during the vapor deposition even though burnout paths covera large area.

According to the invention, the problem is solved by the provision of aninsulating layer over the cooling drum as a barrier resistance which hasan electrical resistance and a thickness so as to assure that a clean,however, limited area of burnout of the coating takes place in the areaof pores or weak points of the plastic foil. A clean burnout in thisinvention means that an electrically conductive connection to a metallayer does not exist or remain through the pores of the plastic foil.

The insulating layer on the cooling drum serves as a barrier resistanceand has a sufficient resistance that means it has a small enoughresistance per unit area relative to the vapor deposited foil to allow alarge portion of the applied voltage to be applied to the dielectricfoil to assure that the foil is attracted and held smooth on the coolingdrum. A high degree of efficiency is achieved when the insulating layeron the drum is selected so that the product of the specific resistanceand its thickness does not exceed a value equal to one-fifth of theproduct of the specific resistance and the thickness of the plasticfoil.

An advantageous embodiment of the inventionlies in the fact that plasticfoil is attractedflto a cooling drum by electrostatic forces in the areawhere vapor deposition of metal occurs and wherein an insulating layerin the form of an insulating foil passes with the plastic foil acrossthe cooling drum. Thus, the advantages achieved that over and over againdifferent areas of the insulating layer come in contact with the drumand with the plastic foil and breakdown channels do not form through theinsulating layer. The insulating layer is strong enough if itsresistance is chosen large enough such that the voltage necessary forthe creation of the electrostatic forces to attract the foil to the drumdoes not cause a breakdown through the insulating layer even in the weakpoints of the foil. The insulating layer does not necessarily have tohave a voltage resistance which by itself prevents the breakdown of thevoltage through the insulating layer but it need only have a resistanceto the applied voltage which is necessary for the prevention ofbreakdowns at the weak points of the foil which has been vaporized.

When breakdowns and burnouts occur, the energy is limited so thatburning out takes place, but excessively large destructionvof thecoating in the area of the breakdowns does not occur. However, theelectrostatic field is maintained at its normal'value so that attractionbetween the foil and the drum does not decrease. On the other hand afterstarting the evaporation the entire electrostatic force should beavailable within a few seconds.

These two conditions allow a time constant to be selected which resultsfrom the product from the absolute dielectric constant and a specificresistance of the insulating layer. It has been discovered that it isdesirable to choose a time constant between 10 microseconds and 1second. Thus, the upper limit is set by the requirement that after thevapor deposition has commenced, the electrostatic field forces shouldbecome fully effective without an unusual delay which means should occurafter a few seconds. The lower limit is set physically in that during apossible breakdown which occurs the charge from the adjacent areas hasto be strongly re tarded and delayed so that only a very small fractionof the energy in the entire capacitor between the coating and thesurface will be supplied during the short duration of the breakdownwhich occurs during a time of approximately 1 microsecond in thebreakdown spark.

In addition, it is advantageous to provide a barrier re sistance in thecurrent feed line such that during short duration of the breakdownspark, energy is not supplied from the voltage source. Since, accordingto the present invention, the number of breakdowns and the total energydissipated during a breakdown is maintained, a high resistance value,'asfor example, 50,000 ohms can be chosen for the barrier resistance.

The insulating layer can be formed on the cooling drum as an endlessbelt which engages the drum at the point and areas where the plasticfoil contacts the cooling drum.

Another arrangement may be made wherein the cooling drum is covered withan insulating layer which might, for example, be a thick cellulosepropionate layer about 10 micrometers in thickness. The insulating layercan be improved so as to taper the amount of energy discharged in thecase of breakdowns if the cylindrical wall of the cooling drum iscomposed of a multitude of small cells which are insulated from eachother and which are connected to each other electrically by a voltagesource via barrier resistances and wherein the barrier resistances'areselected such that the time con- 3 stant for charging them relative tothe vapor deposited coating is between microseconds and 1 second.

In such an arrangement, the electrostatic adhesion between the foil andthe drum can be reversed prior to removing the foil by discharging thearea of the cooling cylinder wall adjacent the point where the foil isto leave the cooling drum. The surface of the cylinder wall of thecooling drum can be preferably designed in rasterlike form or instripes.

Other objects, features and advantages of the invention will be readilyapparent from the following description of certain preferred embodimentsthereof, taken in conjunction with the accompanying drawing, althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWING The FIGURE illustrates the novel methodof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The FIGURE illustrates theplastic foil 1 which passes over a first guide roller 2 and then over acooling drum 3 and over a second guide roller 4. The cooling drum 3 maybe filled with a cooling liquid 8 as shown in cutaway. The outerwallsurface of the cooling drum 3 may be coated and covered with aninsulating layer 6 which has a resistance per unit area which is smallrelative to the resistance per unit area of the plastic foil 1.

A voltage source 10 has one side connected to ground and has itspositive terminal connected to the guide roller 4 and the metalizedlayer of the foil engages the electrically conducting roller 4 so as toapply a positive charge to the metalized side of the foil 1. The coolingdrum 3 has an inner conducting portion which is grounded so thatelectrostatic attraction exists between the foil and the drum so as tohold the foil smoothly onto the drum 3.

In the area where the foil is held smoothly to the cooling drum 3, metalcoating is applied from an evaporator 7 which applies metal vaporthrough the opening 5 in the shutter 9 onto the foil.

The surface of the cooling drum 3 may be formed by several mutuallyinsulated pieces or stripes which are grounded only near the opening 5.

In operation, the rollers 2 and 4 are driven with the roller 4 beingconnected to the positive terminal of the voltage source 10. The roller2 may also be attached to the positive terminal of the voltage source 10so as to apply a positive potential to the foil 1 before it reaches thedrum 3. The cooling drum is grounded and thus, the foil is attracted andheld on the surface of the cooling drum in the metalizing region and ametal layer is deposited from the foil from the evaporator 7 through theopening 5 in the shutter 9. The insulating layer 6 is selected such thatthe produce of its thickness and its resistance per unit area isapproximately one-fifth of the product of the resistance per unit areaand the thickness of the plastic foil 1.

It is seen that this invention provides an improved method for uniformlyand smoothly appling a metal layer to an insulating foil; and althoughit has been described with respect to preferred embodiments, it is notto be so limited as changes and modifications may be made which arewithin the full intended scope as defined by the appended claims.

We claim as our invention:

1. Apparatus for vapor deposition of a metal layer to a plastic foilcomprising a metallic cooling drum rotatably supported, an insulatinglayer covering the cylindrical surface of said cooling drum, meansmoving said plastic foil with one surface entrained about saidinsulating layer on said cooling drum, means for applying a metallizedlayer to the opposed surface of said plastic foil at the area ofentrainment and voltage means for applying an electrostatic fieldbetween said cooling drum and said plastic foil including anelectrically conducting roller engageable with the metallized side ofthe plastic foil.

2. Apparatus according to claim 1, wherein the product of the specificresistance and thickness of said insulating layer on said cooling drumis less than one-fifth of the product of the specific resistance andthickness of said plastic foil.

3. Apparatus according to claim 1 wherein the surface of said coolingdrum is formed with strips so that said insulating layer is divided intosmall areas.

1. Apparatus for vapor deposition of a metal layer to a plastic foilcomprising a metallic cooling drum rotatably supported, an insulatinglayer covering the cylindrical surface of said cooling drum, meansmoving said plastic foil with one surface entrained about saidinsulating layer on said cooling drum, means for applying a metallizedlayer to the opposed surfacE of said plastic foil at the area ofentrainment and voltage means for applying an electrostatic fieldbetween said cooling drum and said plastic foil including anelectrically conducting roller engageable with the metallized side ofthe plastic foil.
 2. Apparatus according to claim 1, wherein the productof the specific resistance and thickness of said insulating layer onsaid cooling drum is less than one-fifth of the product of the specificresistance and thickness of said plastic foil.
 3. Apparatus according toclaim 1 wherein the surface of said cooling drum is formed with stripsso that said insulating layer is divided into small areas.